Category Archives: EuroSun2008-11

Detailed Air-to-Water Heat Exchanger Model for a. Multicomponent Solar Thermal System

Janybek Orozaliev[3] [4]*, Christian Budig1, Klaus Vajen1, Elimar Frank1#,
Ruslan Botpaev[5], Alaibek Obozov2

1 Kassel University, Institute of Thermal Energy Engineering, Kassel (Germany)

2 Kyrgyz State Technical University, Department for Renewable Energies, Bishkek (Kyrgyzstan)
Corresponding Author, www. solar. uni-kassel. de

Abstract

A fin-and-tube heat exchanger model is presented in this paper. It uses empirical heat transfer and flow friction correlations identified in the literature. The model structure, its range of validity and accuracy are described in detail. Additionally, the model performance is compared with the producer design software GPC.

1. Introduction

Fin-and-tube heat exchangers are widely used in industry and residential air conditioning for heat transfer between a liquid and a gas, e. g. for water cooling, air cooling or heating. In the context of a research project in Bishkek (Kyrgyzstan) such a heat exchanger is applied in a multicomponent solar thermal system [1] to heat up cold water for a district heating net by using the enthalpy of the ambient air. The ambient air is also preheated with an unglazed transpired air collector before going through a heat exchanger.

In practice, the heat exchanger geometry is selected by designers relying on their personal experience and some recommendations1. If the heat capacity rate ratio of liquid to air is not defined by the application, a typical value (e. g. 2) is applied. This procedure works well for standard applications. But for non-standard applications with different boundary conditions (e. g. low electricity prices) as described above, there are more detailed investigations necessary. In order to optimize the heat exchanger configuration for this application, a detailed heat exchanger model was developed, which is presented in this paper.

New subsystem based structure level in SIMULATION STUDIO

1.1. Concept

The concept of the new implemented structure level in SIMULATION STUDIO is based on the aim of obtaining a well structured graphical representation of the complete model on the one hand and a high level of modularity and flexibility on the other hand. This is achieved by dividing a model in several subsystems, which can be connected and replaced easily. In this way, modularity is given not only on component but also on subsystem level. A new feature realized within the subsystem approach is the possibility of removing and transferring subsystems without deleting or reconfiguring any visual connections in SIMULATION STUDIO. Fig. 2 shows a sketch of the new subsystem based structure level.

image322

Fig. 2: Subsystem based structure level. Subsystems are connected via uniform defined interfaces only and can be removed and replaced easily. Inside the subsystems several components are connected to each other in the common TRNSYS way. To connect two subsystems with each other only two visual links (one for each

direction) are needed.

1.2. Subsystems

A subsystem is a collection of several components which are connected to each other in the common TRNSYS way. In solar thermal systems the subsystems are conform to the several loops in such a system, e. g. collector loop. Direct connections between components from different subsystems do not occur anymore. Instead of these connections the subsystems are connected via uniform interfaces. This gives the edge of avoiding many overlapping links in the graphical representation. Parameters that are used by the components within the subsystems are defined in separate EQUATION blocks belonging to the respective subsystem. As an example Fig. 3 shows the subsystem of a possible collector loop.

Design tool KOLEKTOR 2.2 for virtual prototyping. of solar Hat-plate collectors

T. Matuska*, J. Metzger and V. Zmrhal

Czech Technical University, Faculty of Mechanical Engineering, Department of Environmental Engineering,

Technicka 4, 166 07 Prague 6, Czech Republic

* Corresponding Author, tomas. matuska@fs. cvut. cz

Abstract

Mathematical model and design software tool KOLEKTOR 2.2 with user-friendly interface for detailed modelling of solar thermal flat-plate collectors is presented. Mathematical model is based on internal and external energy balance of the absorber solved in iteration loops to determine the temperature distribution and heat transfer coefficients in main parts of solar collector. Mathematical model has been validated with experimentally obtained data for different solar flat-plate collector concepts (low and high performance atmospheric collectors, evacuated collector). The software tool KOLEKTOR 2.2 is applicable especially for design and virtual prototyping of new solar flat-plate collectors resulting in efficiency curve determination, for parametric analyses to obtain information on different parameters influence on collector performance and especially for investigation of thermal performance of advanced solar collectors (building integrated, evacuated collectors, etc.).

Keywords: solar collector, evacuated collector, performance modelling, experimental validation

1. Introduction

Computer modelling of solar thermal collectors is a principle approach for testing of new construction concepts and improvements in the development and design stage for developers and manufacturers. Virtual prototyping of solar collectors can save the investments into number of prototypes and foreseen the collector performance in advance. Analyses of individual construction parts and details parameters impact on the collector performance is needed to make decision on efficient solar collector concepts for given application, operation and climatic conditions with respect to economic parameters of construction.

A mathematical model is always a simplification of reality to certain extent. Too complex mathematical models and numerical programs require huge amount of computer time for calculations, too simplified models don’t take important influences of detailed collector parameters into account and result in considerable uncertainty in calculation. To find a good compromise between simplicity of the model and its accuracy is crucial for development of any design and simulation tool.

Although the theory of flat-plate solar collectors is well established and can be found in basic literature [1-3], there is a lack of user-friendly design programs for solar collector performance modelling considering the detailed geometrical and physical parameters of collector. Number of authors evolved simplified analytical models considering temperature independent solar collector overall heat loss coefficient (linear dependence of efficiency), neglecting the absorber temperature distribution or temperature difference between absorber surface and heat transfer fluid. Such

models are not comparable with physical experiments and cannot predict the real performance behaviour and evaluate efficiency characteristics of solar collectors.

Theoretical model of solar collector has been introduced in TRNSYS Type 73 [4] but with simplified calculation of collector heat loss coefficient U insufficient to cover wide range of parameters affecting the collector heat loss. More theoretical model with number of detailed input parameters and calculation of heat transfer coefficients in the individual parts of collector (in air gaps, inside pipes, at outer surfaces) has been evolved as a Type 103 [5].

A design program CoDePro [6] for energy performance calculation of solar flat-plate collectors has been developed with the Energy Equation Solver. It allows a very detailed specification of collector geometrical and material parameters. It covers large segment of solar collectors (unglazed, single and double glazed) and evaluates also optical properties of collector, e. g. incident angle modifier. On the other hand, the features of CoDePro program, analogous to TRNSYS Type 103, don’t allow energy performance modelling of advanced solar collectors, e. g. collectors integrated into building envelope, evacuated flat-plate collectors or solar collector with glazing made of transparent structures.

The presented model and design tool KOLEKTOR 2.2 has been developed to overcome the drawbacks of previous models. KOLEKTOR 2.2 is based on detailed calculation of heat transfer from the collector absorber to ambient and from the collector absorber to heat transfer fluid. The advantage of the design tool is its universality to wide-range of solar flat-plate collectors stock from evacuated to atmospheric, separately or building integrated, covered with different types of glazing (single glazing or transparent insulation structures), etc.

Building modeling. Main characteristics

The analysis will deal with a theoretical building (used in the IEA-ECBCS annex 48 project — Heating Pumping and Reversible Air Conditioning -) fully defined by P. Stabat [3] . It is a twelve identical floors, 15000 m2 building with average 1000 persons occupancy. Representative existing European office building parameters are used [3] . Set points and ventilation rate are defined in table 2. . U value [W/(m2 K)] for external walls is 0.8, 0.4 for roof and 2.95 for glazing.

Ventilation rate (maximum occupation)

25 m3/h/person in offices (1 person/12 m2)

30 m3/h/person in conference room (1 person/3.5 m2) 6h-20h during week — Stopped during weekend

Set point temperatures

21°C — 24°C

inoccupation heating temperature : 15°C

heating from 6h to 20h except Saturday and Sunday

Air conditioning stopped during non occupation period

Table 2. Temperature set points and ventilation rate

In TRNSYS, only one floor with 5 thermal zones is simulated, there is no heat transfer considered between floors.

Study of Heat Load in Simulation of Solar Hot Water Heating Systems

T. Kusunoki1* and M. Udagawa1

1Kogakuin University, 1-24-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo, 163-8677, JAPAN
Corresponding Author, dd08002@ns. kogakuin. ac. jp

Abstract

As the performance of solar hot water heating system is strongly influenced by heat load, this study is focused on the followings.

1) Usually the simulation of the solar hot water heating system is carried out with 1 hour time increment. However, the domestic hot water demand varies in very short time. In this study, the simulation was carried out for considering the suitable simulation time increments in solar hot water heating systems. As a result, it was found that 10 minutes is the suitable time increment for the simulation of the solar hot water heating system.

2) Using the time increment of 10 minutes, the simulation for 42 combinations of collector tilt angle and azimuth for five cities in Japan was carried out. As a result, it was found that the difference in the performance of solar hot water heating systems was within 15% from the best performance with the combination of the tilt angle from10 to 60 degrees and azimuth within 45 degrees from the south in Tokyo. The similar results were obtained for the combination of the tilt angle and azimuth in other four cities. The simulation result showed that the solar contribution for DHW heating load in Sapporo, Morioka, Sendai, Tokyo and Kagoshima were 44 %, 46 %, 51 %, 57 % and 66 %, respectively. The difference is caused by the DHW heating load which is strongly influenced by the city water temperature.

Keywords: Solar DHW System, Simulation Time Increments, Tilt Angle, Azimuth

1. Introduction

More than 35% of the purchased energy consumption is used for domestic hot water (DHW) heating in the Japanese housing sector. Therefore, it is important to reduce the energy consumption used for the hot water heating. Reduction of the energy consumption by solar DHW heating system is expected in the housing sector.

Solar DHW heating systems are affected by the DHW use profile as well as the solar radiation on collector. As the DHW use varies in very short time, the simulation of the solar DHW heating systems needs to consider for suitable time increments in the simulation. In this study, the time increment in the simulation of the solar DHW heating systems to reflect the detailed hot water use profile was examined first.

The overall solar system performance was also studied to find the allowable collector tilt angle and azimuth in designing the solar DHW heating system. In addition, the total performance of the solar DHW system is compared for five cities in Japan.

Purpose and fin optimisation parameters

The CFD analysis of the ICS-SWH was undertaken in order to improve the four fin ICS-SWH performance by optimising the fin spacing. Prior to simulation issues regarding the parameters influencing fin optimisation need to be determined, constrains need to be stated, heat transfer parameters need to be outlined and the type of CFD analysis need to be established.

Fin material, length and thickness and the number of fins in the ICS-SWH are the four main parameters to be outlined when analysing fin optimisation. The material thermal conductivity is an intrinsic parameter for effective ICS-SWH. The low density, high thermal conductivity and recyclable properties of aluminium highlighted this material as an effective choice. The fin length was fixed at the maximum length of 800mm. Fins are used to increase the heat transfer from the heated surface by increasing the effective surface area. The effectiveness of the fin is enhanced by increasing the ratio of the perimeter to the cross-sectional [7] therefore the use of thin, but closely spaced fins is preferred, with the provision that the fin gap is not reduced to a value where flow between the fins is severely impeded, thereby reducing the convection coefficient. The present study thus looks at the increase in performance of the ICS-SWH by increasing the number of fins.

Three main constraints affect this study; cost, volume (50litre tank size) and manufacturing ability which need to be borne in mind when choosing the new design.

Three main heat transfer parameters influence the ICS-SWH performance and can be recapitulated as: the shape of fins, the angle of inclination of the heater, and time of exposure to incident solar radiation. Due to manufacturing constraints, simple rectangular shapes of fins were considered. A 45 degree inclination angle and a 300W/m2 heat flux were taken as the reference conditions [6].

Finally, the type of CFD analysis, 2D or 3D, is an important parameter to consider. Previous studies [9] outlined that 2D analysis was sufficient for a good analysis of the system. However, 2D analysis would only suffice for a horizontal inclination of the heater. For any angle above zero a gradient exists in the longitudinal direction making a 2D analysis insufficient. Hence, based on a 45 degree inclination of the collector, a 3D analysis was undertaken. As the Quiescent fluid is unavailable in CFD simulation, the process was assumed transient.

Mathematic description

The theoretical effort invested in modeling the CPV receiver with solar cells immersed in liquid has not seen the intensive research and development activity. The studies of thermal model of solar cells have been mostly concentrated on photovoltaic system and hybrid photovoltaic thermal system. Radziemska [9] summarizes the recent progress obtained in the field of the temperature performance of crystalline and amorphous silicon solar cells and modules. Various authors have modeled the temperature of a PV module by evaluation of energy inputs and outputs through radiation, convection, conduction and power generated. The energy balance of photovoltaic cells is modeled based on climate variables by Jones et al. [10]. Module temperature change is shown to be in a non-steady state with respect to time. A one dimensional heat transfer model was derived by Davis et al [11] to improve upon the NOCT model. Garg et al [12,13] have developed a computer simulation model for predicting the steady state and transient performance of a conventional photovoltaic/thermal (PV/T) air heating collector with single — and double-glass configurations. Lee et al. [14] concerns the development of a thermal model to predict the temperature profile of a typical building integrated PV roof and comparison of the performance of this model to that of the simplified model for flat-plate PV arrays presented by Fuentes.

In the present case, the energy flows among the elements of the CPV receiver and the surroundings are described in the thermal network drawn in Fig. 2. The goal of this network is to evaluate the solar cells temperature immersed in silicone oil. The solar cells temperature is critical to estimating the electrical production.

Program of Energy Development for States and Municipalities — PRODEEM

The Northeast of Brazil, made up of 8 states, represents 18% of the total area of the country, and is responsible for 16% of the production and it has a population of 42,000,000 people (28%). Approximately half of this area, 760,000 km2, is semi-arid where 17,000,000 people live. The climate of this region is hot and dry having a mean annual temperature of 27 °C and 2500 h/year of insolation. The annual precipitation varies from 400 to 800 mm, contrasting with an evapotranspiration of 2500 mm/year, which determines a dry period of more than 7 months. The vegetation that covers the semi-arid region is a deciduous tropical forest, locally known as the caatinga, that develops over a complex mosaic of soils, Fig.1. High level of insolation, scarce hydric resources and rare rainfall, poorly destributed over time, cause long periods of drought. Thus, the relative shortage of superficial hydric resources, make evident the importance of subterranean waters. The exploration of these waters is found to be limited because of the nature of its soils (predominanthy crystalline), low discharge rate ( mean of 3000 l/h) and mainly for its

Подпись: Figure 1 - The Caatinga in dry and rainy periods and details of vegetation

quality. The great majority of the wells present higher salinity indices than the maximum limit permitted for human consumption, that is of 1000 ppm of dissolved total solids and in many cases above 6000 ppm, the extreme water salinity level for animal consumption. Another aggravating factor that makes the solution to the problem of water supply difficult is the low index of rural electrification in the Northeast.

To face this kind of challenge, the Federal Government established the PRODEEM — Program of Energy Development for States and Municipalities through a presidential decree in December 1994, with the following objectives:

• Make viable the installation of energy microsystems for production and local use in isolated needy communities that are not served by electric network, that are destined to aid in attending basic social demands;

• Promote the utilization of decentralized energy sources in the supply of energy to the small producers, to nuclei of colonization and isolated populations;

• Compliment the conventional energy system offer with the use of decentralized renewable energy sources;

• Promote the utilization of human resources and development of technology and national industry, that are essential for implantation and operational continuity of the implanted systems.

Since the year 1996 the PRODEEM has bought and installed thousands of photovoltaic systems that are spread around the national territory. In the phases denominated I to V and a special one denominated Pumping, three kinds of autonomous photovoltaic systems were installed: photovoltaic systems for generation of electric energy (energetic), photovoltaic systems for

Подпись: Figure 2 - Settlement of landless people called Gualter, in the municipality of Caninde of Sao Francisco in Alagoas, after the construction of the water supply system

pumping water and for public illumination. The systems were only destined for application in communities, which means that they should benefit the communities as a whole, and not only some individuals in particular. The total of photovoltaic systems installed in this program was 8,956, corresponding to a total power of 5.2 MWe, and it can be considered as one of the largest rural eletrification programs that use photovoltaic solar energy in developing countries. Most of the photovoltaic systems were installed in the North Northeast regions, 1,471 and 4,577 systems respectively. In the target regions of this project AL, PE and PB 905 systems were installed, in accordance with the MME Eletric Energy Atlas (2008).

Fig. 2 shows the settlement of the landless people called Gualter, in the municipality of Caninde of Sao
Francisco in Alagoas, after the construction of the water supply system all supplied by photovoltaic solar
energy: the water supply system, the collective laundry and the drinking trough for animals. Fig. 3 shows
some other productive applications in community with photovoltaic solar energy: electrified fence and
school. The experience accumulated by the PRODEEM in the last ten years shows the enormous challenge
of management, planning, training and sociology of the implantation, on a large scale and in large territorial
extensions, of innovating technology and decentralized technology as of photovoltaic generation. The
traditional tools demonstrate their limitations and make the execution of such tasks very difficult.

Turbulence model applied to a fluid in a modified evacuated solar collector

L. Crema1*, G. Cicolini1, A. Bozzoli1

1 Renewable Energies & Environmental Technologies research unit (REET), Fondazione Bruno Kessler (FBK),
Via alla Cascata 56/C, I-38050 Povo (Tn), Italy
* Corresponding Author, :rema@,fbk. eu

Abstract

In this work, we propose a solution to convection and conduction heat losses on an solar thermal collector. We have based our work starting from the technology of the evacuated solar tubes, the technology actually with the highest thermal efficiency in the market. The tubes have got a CERMET absorbing layer sputtered on the surface of a glass tube, in the evacuated area. A modified structure for the evacuated tube has been analysed in order to investigate the heat transfer and the thermal resistance from the cermet layer to the vector fluid in different fluid dynamic conditions than the actual used. Some models have been created and their behaviour has been verified using Finite Element Modelling software. The computed results show a lower thermal resistance for the proposed geometries. A higher convective heat transfer has been obtained providing the vector fluid of a turbulent flow, using special shapes (turbulators) applied to the evacuated tubes. The temperature gradient between the cermet layer and the vector fluid has been decreased from 20 to 1,06 K in the better case. The thermal efficiency of the panel has an improvement of about 10% from such modified geometry.

Keywords: solar collector, finite element modelling, fluid dynamic

1. Introduction

During last years the development of new technologies have taken solar thermal collectors to new frontiers [1]. Collectors based on evacuated solar tubes, cermet layers and other applied technologies have taken good results. Many studies had proposed solutions to the evacuated solar tube development [2]. Nevertheless commercial collectors frequently suffer from some problems related to heat transfer phenomena, radiation loss at high temperatures or vector fluid choice. A relevant heat loss comes from conduction/convection phenomena between the absorber material and the vector fluid and from both of them to the environment. In the actual work it has been performed an analysis on a real evacuated solar tube with a relevant presence on the international markets and particularly good characteristics. The aim is to analyse aspects related to heat losses and to propose both a partial solution to the problem and a technical improvement of the system. The analysis will be performed under an analytical point of view, by the use of FEM (Finite Element Modelling) simulation and by an experimental setup for a direct verification of modelling.

Description of global sensitivity analysis methods

The sensitivity analysis methods are divided in local and global ones. With local sensitivity analysis, the influence of parameters could be estimated only around a certain point in the parameters space. In the case of solar heating system, such an estimation is needed in a wider range of parameters

variations, in a certain volume of space. For this, two representatives of global sensitivity analysis are applied, namely the Morris method and the Fourier amplitude sensitivity test (FAST).